This invention relates to an image reading apparatus for use in a facsimile machine, a copying machine, and the like.
In an image reading apparatus for use in a facsimile machine and a copying machine, a white reference as a reference of an image density is read by the use of a white reference plate before reading an original image. In presence of dust or dirt on the white reference plate, a shading correction waveform becomes abnormal. In this event, a white line is inevitably produced when the original image is read.
In order to solve the above-mentioned problem, proposal is made of a shading correction circuit disclosed in Japanese Unexamined Patent Publication No. H07-264400 (JP 7-264400A) which will hereinafter be referred to as a first conventional technique.
Referring to FIG. 1, the shading correction circuit according to the first conventional technique comprises an image sensor 1 for reading an original image to produce analog image data, an A/D (analog to digital) converter 2 connected to the image sensor 1 for converting the analog image data into digital image data, and a switch 3 connected to the A/D converter 2. Upon factory shipment, initial shading waveform data are acquired. Specifically, the image sensor 1 reads a white reference plate as a white reference to produce analog white reference data. The A/D converter 2 converts the analog white reference data into digital white reference data. Through the switch 3, the digital white reference data are supplied to a correction coefficient converting circuit 4 to be converted into a first correction coefficient per each bit in a main scanning direction. The first correction coefficient is sent from the correction coefficient converting circuit 4 through a switch 15 to a first correction coefficient memorizing circuit 16 to be memorized therein. Immediately before reading an original image, the white reference plate is similarly read so that a second correction coefficient is obtained by the correction coefficient converting circuit 4. The second correction coefficient is sent through the switch 15 to a second correction coefficient memorizing circuit 17 to be memorized therein. Supplied with the first and the second correction coefficients memorized in the first and the second correction coefficient memorizing circuits 16 and 17, a white reference contamination judging circuit 18 compares, per each bit, a difference between the first and the second correction coefficients with a predetermined threshold value to judge presence/absence of contamination of the white reference. If the difference between the first and the second correction coefficients is not greater than the threshold value and the white reference therefore includes no contamination, a correction coefficient calculating circuit 19 updates the content (i.e., the first correction coefficient) of the first correction coefficient memorizing circuit 16 with the content (i.e., the second correction coefficient) of the second correction coefficient memorizing circuit 17. On the other hand, if the difference between the first and the second correction coefficients is greater than the threshold value and the white reference therefore includes some contamination with respect to a particular bit, the correction coefficient calculating circuit 19 updates the content (i.e., the first correction coefficient) of the first correction coefficient memorizing circuit 16 with reference to the first and the second correction coefficients memorized in the first and the second correction coefficient memorizing circuits 16 and 17 for an uncontaminated adjacent bit and the first correction coefficient memorized in the first correction coefficient memorizing circuit 16 for the particular bit. Upon reading an original image, a correction coefficient multiplying circuit 8 multiplies digital image data as an output of the A/D converter 2 by the first correction coefficient memorized in the first correction coefficient memorizing circuit 16. In FIG. 1, a reference numeral 20 represents a correction coefficient ratio memorizing circuit.
As described above, the first conventional technique requires the two correction coefficient memorizing circuits 16 and 17 in order to detect the contamination of the white reference. The first correction coefficient memorizing circuit 16 memorizes the first correction coefficient obtained upon the factory shipment of an apparatus comprising the shading correction circuit. The second correction coefficient memorizing circuit 17 memorizes the second correction coefficient obtained immediately before reading the original image. By comparing the difference between the first and the second correction coefficients memorized in the first and the second correction coefficient memorizing circuit 16 and 17 with the threshold value, judgment is made of presence/absence of the contamination of the white reference. In absence of the contamination of the white reference, the content (i.e., the first correction coefficient) of the first correction coefficient memorizing circuit 16 is updated by the content (i.e., the second correction coefficient) of the second correction coefficient memorizing circuit 17. In presence of the contamination of the white reference, the content (i.e., the first correction coefficient) of the first correction coefficient memorizing circuit 16 is updated with reference to the first and the second correction coefficients memorized in the first and the second correction coefficient memorizing circuits 16 and 17 for the uncontaminated adjacent bit and the first correction coefficient memorized in the first correction coefficient memorizing circuit 16 for the particular bit.
In the first conventional technique, however, the first correction coefficient is updated into a new value one after another by the use of a previous value of the first correction coefficient and the second correction coefficient obtained immediately before reading the original image. Therefore, once an incorrect value is memorized due to signal noise or misjudgment upon the contamination of the white reference, the first correction coefficient is continuously updated into an incorrect new value thereafter. The white reference is at first acquired upon the factory shipment. Therefore, in case where dust is attached to the white reference plate itself in a production process, correction is impossible. Such white reference plate can not be used because it is defective. This deteriorates the yield of production of the white reference plate.
As a second conventional technique, proposal has been made of an image reading apparatus disclosed in Japanese Unexamined Patent Publication No. H09-247445 (JP 9-247445 A). The second conventional technique describes that generation of an unnecessary line in an acquired image in presence of dust attached to a first mirror close to a surface of an original image is prevented by shading correction like in the first conventional technique. In the second conventional technique, correction is carried out when reading a multi-value image by comparing white reference data upon shading correction and image data upon reading the image. Therefore, in case where the white reference itself includes dust or dirt, a corresponding bit can not be corrected. Thus, according to the second conventional technique, correction is possible if the dust is attached to the mirror but is impossible if the white reference plate itself is contaminated.
Fine dust on the white reference plate is inevitable in its production process. If the resolution of the image reading apparatus is improved, such fine dust is read and manifested or expressed as a white line in image information.